"What happens when you're in your late 70s and the rug is pulled out from under you?" poses Fonda.

The answer they and their show are promoting: "There can be a third act that's pretty robust and pretty fun," as Fonda puts it. "Don't write us off just because we're over the hill. 'Cause there's a lot of other hills still to come that are pretty exciting."

As the third season begins, the often-fractious friendship shared by uptight Grace and free-spirited Frankie has steered them into marketing a product designed to meet the special needs of women of their vintage. It's a vibrator with thoughtfully large-print directions and a swiveling head that won't aggravate the user's carpal tunnel syndrome or arthritis. Its brand name: Menage a Moi.

Thus does "Grace and Frankie," between generous laughs, continue to explore growing older while thumbing its nose at Father Time.

Fonda and Tomlin (today hale and hearty at, respectively, 79 and 77) are the best advertisements for what the series is about. They first worked together in a certain hit film nearly four decades ago.

Fonda had attended a live performance by Tomlin "and I fell in love with her. I was preparing a movie called '9 to 5' that was kind of serious. But after I saw her show, I thought, 'I CAN'T do a movie about secretaries if Lily Tomlin isn't one of them.' And we had to totally redo the movie so it was funny."

Tomlin says she came to the project "totally in awe" of Fonda, and, referring to Fonda's Oscar-winning performance in the 1971 thriller "Klute," confides, "I had already worn a 'Klute' hairdo for a couple of years."

With their on-screen reunion for "Grace and Frankie," Fonda says Tomlin, once again, is "good for me to be around. I come from a long line of depressed people, and Lily's humor is right close to the surface. I tend to depression, and she finds laughter in everything. We have fun together."

In the first season, Fonda unexpectedly identified with the abandonment issues her character was going through. "It kind of shatters" Grace, she says, and "it did for me, Jane, as well. It triggered something in me. It was really hard to be playing somebody going through that."

And there are other demands besides the emotional. Like "working 16 hours a day when you're almost 80," Fonda says. "And we have to be learning our lines. Some of us," she adds with a comically knowing eye in Tomlin's direction, "have QUITE a challenge that way."

"I have NO problem learning lines!" Tomlin chortles. "She is just so full of it!"

"Lily stumbles around and forgets her lines," persists Fonda, "then, when it's all put together, it all turns out perfectly. She somehow makes it seem so fresh. When I forget MY lines, I just feel sort of like I'm a dud."